Large cooling installations, such as industrial refrigeration systems or air conditioner systems for office complexes, often involve the use of high cooling capacity systems of greater than 400 refrigeration tons (1400 kW). Delivery of this level of capacity typically requires the use of very large single stage or multi-stage compressor systems. Existing compressor systems are typically driven by induction type motors that may be of the hermetic, semi-hermetic, or open drive type. The drive motor may operate at power levels in excess of 250 kW and rotational speeds in the vicinity of 3600 rpm. Such compressor systems typically include rotating elements supported by lubricated, hydrodynamic or rolling element bearings.
The capacity of a given refrigeration system can vary substantially depending on certain input and output conditions. Accordingly, the heating, ventilation and air conditioning (HVAC) industry has developed standard conditions under which the capacity of a refrigeration system is determined. The standard rating conditions for a water-cooled chiller system include: condenser water inlet at 29.4° C. (85° F.), 0.054 liters per second per kW (3.0 gpm per ton); a water-side condenser fouling factor allowance of 0.044 m2-° C. per kW (0.00025 hr-ft2-° F. per BTU); evaporator water outlet at 6.7° C. (44.0° F.), 0.043 liters per second per kW (2.4 gpm per ton); and a water-side evaporator fouling factor allowance of 0.018 m2-° C. per kW (0.0001 hr-ft2-° F. per BTU). These conditions have been set by the Air-Conditioning and Refrigeration Institute (ARI) and are detailed in ARI Standard 550/590 entitled “2003 Standard for Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle,” which is hereby incorporated by reference other than any express definitions of terms specifically defined. The tonnage of a refrigeration system determined under these conditions is hereinafter referred to as “standard refrigeration tons.”
In a chiller system, the compressor acts as a vapor pump, compressing the refrigerant from an evaporation pressure to a higher condensation pressure. A variety of compressors have found utilization in performing this process, including rotary, screw, scroll, reciprocating, and centrifugal compressors. Each compressor has advantages for various purposes in different cooling capacity ranges. For large cooling capacities, centrifugal compressors are known to have the highest isentropic efficiency and therefore the highest overall thermal efficiency for the chiller refrigeration cycle. See U.S. Pat. No. 5,924,847 to Scaringe, et al.
Typically, the motor driving the compressor is actively cooled, especially with high power motors. With chiller systems, the proximity of refrigerant coolant to the motor often makes it the medium of choice for cooling the motor. Many systems feature bypass circuits designed to adequately cool the motor when the compressor is operating at full power and at an attendant pressure drop through the bypass circuit. Other compressors, such as disclosed by U.S. Pat. No. 5,857,348 to Conry, link coolant flow through the bypass circuit to a throttling device that regulates the flow of refrigerant into the compressor. Furthermore, U.S. Patent Application Publication 2005/0284173 to de Larminat discloses the use of vaporized (uncompressed) refrigerant as the cooling medium. However, such bypass circuits suffer from inherent shortcomings.
Some systems cool several components in series, which limits the operational range of the compressor. The cooling load requirement of each component will vary according to compressor cooling capacity, power draw of the compressor, available temperatures, and ambient air temperatures. Thus, the flow of coolant may be matched properly to only one of the components in series, and then only under specific conditions, which can create scenarios where the other components are either over-cooled or under cooled. Even the addition of flow controls cannot mitigate the issues since the cooling flow will be determined by the device needing the most cooling. Other components in the series will be either under-cooled or over cooled. Over cooled components may form condensation if exposed to ambient air. Under-cooled devices may exceed their operational limits resulting in component failure or unit shut down. Another limitation of such systems may be a need for a certain minimum pressure difference to push the refrigerant through the bypass circuit. Without this minimum pressure, the compressor may be prevented from operating or limited in the allowed operating envelope. A design is therefore desired which provides the capability for a wide operating range.
Centrifugal compressors are also often characterized as having undesirable noise characteristics. The noise comes from the wakes created by the centrifugal impeller blades as they compress the refrigerant gas. This is typically referred to as the “blade pass frequency.” Another source of noise is the turbulence present in the high speed gas between the compressor and the condenser. Noise effects are particularly prevalent in large capacity systems.
Another characteristic of existing large capacity centrifugal compressors designs is the weight and size of the assembly. For example, the rotor of a typical induction motor can weigh hundreds of pounds, and may exceed 1000 pounds. Compressor assemblies having capacities of 200 standard refrigeration tons can weigh in excess of 3000 pounds. Also, as systems are developed that exceed existing horsepower and refrigerant tonnage capacity, the weight and size of such units may become problematic with regard to shipping, installation and maintenance. When units are mounted above ground level, weight may go beyond problematic to prohibitive because of the expense of providing additional structural support. Further, the space needed to accommodate one of these units can be significant.
There is a long felt need in the HVAC industry to increase the capacity of chiller systems. Evidence of this need is underscored by continually increasing sales of large capacity chillers. In the year 2006, for example, in excess of 2000 chiller systems were sold with compressor capacities greater than 200 standard refrigeration tons. Accordingly, the development of a compressor system that overcomes the foregoing problems and design challenges for delivery of refrigeration capacities substantially greater than the existing or previously commercialized systems would be welcome.